This invention concerns filters, and more particularly filters of the type comprising a drum carrying one or more filtering media layers through which the liquid to be filtered is directed. Disclosed in each of U.S. Pat. Nos. 4,407,720; 4,622,145; and 4,735,730 is a filter comprised of a rotatable drum disposed in a tank containing the liquid to be filtered. The drum is formed by a circumferential array of axially extending bars, the spaces between which form flow paths for liquid after it has passed through the filter media layer disposed about the outside of the bars. The filter media layer is also disposed about the inside of the drum in the vertical axis drum filter shown in U.S. Pat. No. 4,622,145.
The filter media layer in these patents is disclosed as a series of windings of wedge shaped wire, which are spaced apart to create filtering openings which increase in size in the direction of flow. The wedge wire filter media is advantageous in that the diverging shape tends to prevent clogging by particles wedging in the openings.
The filter openings so formed are slit shaped since they are defined by the gap between successive windings of the wedge wire. In certain filter applications such as removing machined aluminum chips, flake shaped solids must be removed from the liquid.
In those applications, an unacceptable proportion of the solid flakes will sometimes pass through the slit shaped openings, and thus are present in the filtered liquid to an excessive level.
If the spacing is made very close to prevent the passage of these flakes, gelatinous material such as "tramp oil" collecting in the system will clog the opening.
While a perforated plate or wire mesh filter media layer could be employed to establish a less elongated shaped opening, and sized to prevent passage of all of the filtered solid particles, this approach would lose the self-cleaning feature of the wedge-shaped filtering openings. Also, the uneven surface of a wire mesh would prevent effective scraping of the surface to remove accumulated solids.
Even with wedge-shaped openings, filter media will sometimes tend to pack with solids and become clogged, and it would be advantageous to be able to unclog such openings during periodic cleanings.
Scraping of the filter surface is often done to remove a heavy accumulation of solids on the surface, but this is not usually effective to clear the filter media openings, particularly if finely sized spaces are employed in an effort to remove the thin flakes described above.
Backwashing is also performed, which is effective, but less so on the wedge-shaped filtering openings, which tend to pack with the reverse backwashing flow.
Accordingly, it is an object of the present invention to provide a drum filter in which the tendency for flake-shaped solids to pass through the filter media layers is minimized.
It is a further object of the present invention to provide a drum filter in which the clogging of the openings may be effectively cleared by a scraping operation.
Another object of the present invention is to provide a method of manufacturing a drum filter which incorporates a very thin porous sheet layer as the filter media.